Solid oxide fuel cells (SOFCs) are multi-layered structures formed primarily of high-purity metal oxides, including an ionic conducting electrolyte, which generate electricity from the electrochemical oxidation of a fuel source. Planar SOFC configurations are relatively simple to manufacture and have greater power densities and efficiencies than other configurations, but require hermetic seals to prevent mixing of the fuel and oxidant streams within the cell stack and to seal the stack to the system manifold.
The seals must have a low electrical conductivity and must be chemically and mechanically stable in a high temperature reactive environment (moist reducing and/or oxidizing conditions). The seals should exhibit no deleterious interfacial reactions with other cell components, should be created at a low enough temperature to avoid damaging cell components (under 900° C. for some materials), and should not migrate or flow from the designated sealing region during sealing or cell operation because of any applied load.
In addition, the sealing system should be able to withstand thermal cycling between the operational temperature and room temperature. That is, thermal stresses that develop because of mismatches in the thermal contraction characteristics of the different SOFC materials must either be reduced to well below the failure strengths of the materials or must be relieved in some fashion. Although it is possible to design rigid glass-ceramics with coefficient of thermal expansion (CTE) characteristics that are compatible with other SOFC materials (e.g., yttria-stabilized zirconia (YSZ), ferritic stainless steels such as SS441, and alumina (as a coating material on ferritic steels)), and are stable over a long period of time at the operational temperature, stresses can still develop because of in-plane temperature gradients during operation and thermal cycling. If these stresses lead to cracks in the rigid glass seal or at one of the seal interfaces, the operational integrity of the SOFC is compromised.
Currently available glass seals for SOFCs are mostly based on glass-ceramics which turn into a rigid ceramics after crystallization at the SOFC operational temperature, 650-850° C. These rigid glass seals may have intrinsic flaws that are hard to eliminate and can be detrimental when CTE is not matched. Compliant glass seals have been developed as a means to overcome the limitations of the rigid sealants. These glass seals, however, contain alkali elements that may cause undesirable reactions with other SOFC components, or contain expensive precious metals such as silver. Attempts have been made to develop viscous glass sealants for SOFCs, but these attempts use gallium and/or germanium that may limit the commercialization due to their high costs.